1. Field of the Invention
The present invention relates to a multilayer wiring board and a manufacturing method therefor. More particularly, the present invention relates to a multilayer wiring board comprising a plurality of insulating substrates laminated together to form a multilayer substrate; a plurality of circuit patterns formed on both surfaces or one surface of each of the insulating substrates, the multilayer substrate being formed with a plurality of blind holes having different depths and/or a through hole extending selectively through the circuit patterns and the insulating substrates; and a conductive film formed on an inner circumferential surface and a bottom surface of each of the blind holes and/or an inner circumferential surface of the through hole, for electrically connecting the circuit pattern perforated to the circuit pattern exposed to the bottom surface of each of the blind holes and/or electrically connecting the circuit patterns perforated to each other. The present invention also relates to a manufacturing method of such a multilayer wiring board.
2. Description of Related Art
In recent years, electronic equipments tend to be compact and multifunctional, and in association therewith, a high-density construction of wiring boards to be used in the electronic equipments has been progressed. The high-density construction has been realized by a so-called multilayer wiring board such that a plurality of circuit patterns are formed not only on both surfaces of a multilayer substrate constructed of a plurality of insulating substrates but also on each insulating substrate forming an inner layer, and the circuit patterns in different layers of the multilayer substrate are electrically connected with each other.
The electrical connection between the circuit patterns in different two layers is realized by forming a through hole extending through the two circuit patterns and the insulating substrate interposed therebetween or forming a blind hole extending through one of the two circuit patterns and the insulating substrate and having a bottom exposed to the other circuit pattern, and by forming a conductive film on an inner circumferential surface of the through hole or on both an inner circumferential surface and the bottom of the blind hole. It is appreciated that the smaller the diameter of the through hole or the blind hole, the higher the wiring density of the wiring board. However, it is technically difficult to reduce the diameter of the through hole or the blind hole. Practically, it is impossible to form the blind hole having a diameter of 0.3 mm or less by drilling.
Another method of forming such a hole by blasting an abrasive powder against the multilayer substrate is described in Japanese Patent Application No. 2-135665 filed by the present applicant.
FIGS. 4 and 5A to 5F show a multilayer wiring board and a manufacturing method therefor as described in this reference.
Referring to FIG. 4, reference character a generally. denotes a multilayer wiring board; b.sub.1, b.sub.2 and b.sub.3 denote insulating substrates; c.sub.0, c.sub.1, c.sub.2 and c.sub.3 denote circuit patterns respectively formed from copper foils g.sub.0, g.sub.1, g.sub.2 and g.sub.3 on the insulating substrates b.sub.1, b.sub.2 and b.sub.3.
Reference characters d denote conductive films formed to provide electrical connection among the circuit patterns c.sub.0 to c.sub.3 through the different layers. A blind hole e.sub.1 is formed through the circuit pattern c.sub.0 and the insulating substrate b.sub.1. Another blind hole e.sub.2 is formed through the circuit pattern c.sub.0 and the insulating substrates b.sub.1 and b.sub.2. A through hole f is formed through the circuit patterns c.sub.0, the insulating substrates b.sub.1, b.sub.2 and b.sub.3, and the circuit pattern c.sub.3. The conductive films d are formed on the inner circumferential surfaces of the blind holes e.sub.1 and e.sub.2 and the through hole f and on the connecting portions of the circuit patterns c.sub.O to c.sub.3.
The multilayer wiring board a having such a construction is manufactured in the following manner.
First, the insulating substrate b.sub.2 having both surfaces on which the copper foils g.sub.1 and g.sub.2 are laminated is prepared (see FIG. 5A), and the copper foils g.sub.1 and g.sub.2 are partially removed by photoetching to form the predetermined circuit patterns c.sub.1 and c.sub.2 (see FIG. 5B). Then, the other insulating substrates b.sub.1 and b.sub.3 are laminated on both surfaces of the insulating substrate b.sub.2 so as to fully cover the circuit patterns c.sub.1 and c.sub.2, and then the copper foils g.sub.0 and g.sub.3 are laminated on both surfaces of such a laminated body constructed of the insulating substrates b.sub.1, b.sub.2 and b.sub.3, thus forming a multilayer substrate h (see FIG. 5C).
Next, the blind holes e.sub.1 and e.sub.2 and the through hole f are formed at predetermined positions of the multilayer substrate h in accordance with the following steps.
The blind holes e.sub.1 and e.sub.2 are different in depth from each other. The blind hole e.sub.1 extends through the copper foil g.sub.0 (to be formed into the circuit pattern c.sub.0) formed on the upper surface of the multilayer substrate h and further extends through the insulating substrate b.sub.1 of the first layer in such a manner that the circuit pattern c.sub.1 (formed from the copper foil g.sub.1) formed on the upper surface of the insulating substrate b.sub.2 is exposed to the bottom of the blind hole e.sub.1. The blind hole e.sub.2 is deeper than the blind hole e.sub.1, and extends through the copper foil g.sub.0, the insulating substrate b.sub.1 of the first layer, and the insulating substrate .sub.2 of the second layer in such a manner that the circuit pattern c.sub.2 (formed from the copper foil g.sub.2) formed on the lower surface of the insulating substrate b.sub.2 is exposed to the bottom of the blind hole e.sub.2.
The blind holes e.sub.1 and e.sub.2 and the through hole f are formed at predetermined positions of the multilayer substrate h in accordance with the following steps.
First, a mask j having small apertures i.sub.1, i.sub.2 and i.sub.3 of a predetermined size identical in diameter are formed on one surface of the multilayer substrate h on the side where the blind holes e.sub.1 and e.sub.2 are to be formed (see FIG. 5D). The apertures i.sub.1, i.sub.2 and i.sub.3 of the mask j are registered with the forming positions of the blind holes e.sub.1 and e.sub.2 and the through hole f, respectively. Then, an abrasive powder (not shown) is blasted against the upper surface of the multilayer substrate h on which the mask j has been formed, so as to etch the portions of the multilayer substrate h corresponding to the apertures i.sub.1, i.sub.2 and i.sub.3.
The mask j to be employed herein is formed of such a material as not to be etched by the blasting of the abrasive powder. For example, a photosensitive urethane rubber is preferably used.
The mask j is formed by first applying a photosensitive material on the entire surface of the multilayer substrate h on the side where the blind holes e.sub.1 and e.sub.2 are to be formed later, then irradiating ultraviolet rays through a photomask to the coating of the photosensitive material, and then selectively removing the portions of the coating to be formed into the apertures i.sub.1, i.sub.2 and i.sub.3.
When the abrasive powder is blasted against the upper surface of the multilayer substrate h on which the mask j has been formed, the copper/oil g.sub.0 exposed to the first aperture i.sub.1 of the mask j and the insulating substrate b.sub.1 disposed under the copper foil g.sub.0 are etched by the abrasive powder to form the blind hole e.sub.1 ; the copper foil g.sub.0 exposed to the second aperture i.sub.2 of the mask j, the insulating substrate b.sub.1 disposed under the copper foil g.sub.0, and the insulating substrate b.sub.2 disposed under the insulating substrate b.sub.1 are etched by the abrasive powder to form the blind hole e.sub.2 ; and the copper foil g.sub.0 exposed to the third aperture i.sub.3 of the mask j, the insulating substrates b.sub.1, b.sub.2 and b.sub.3, and the copper foil g.sub.3 disposed under the insulating substrate b.sub.3 are etched by the abrasive powder to form the through hole f (see FIG. 5E).
Thereafter, copper plating is carried out on both surfaces of the multilayer substrate h, the inner circumferential surfaces and the bottom surfaces of the blind holes e.sub.1 and e.sub.2, and the inner circumferential surface of the through hole f, thereby forming the conductive film d for effecting electrical connection among the copper foils g.sub.0 and g.sub.3 and the circuit patterns c.sub.1 and c.sub.2 (see FIG. 5F).
Finally, the copper foils g.sub.0 and g.sub.3 formed on the upper and lower surfaces of the multilayer substrate h are partially removed together with the conductive film d by photoetching to thereby form the circuit patterns c.sub.O and c.sub.3. Thus, the multilayer wiring board a shown in FIG. 4 is completed.
However, the multilayer wiring board and the manufacturing method therefor in the prior art as mentioned above have the following defects.
First, in the plating step wherein a plating liquid is admitted into the blind holes e.sub.1 and e.sub.2 and the through hole f, the plating liquid is hard to penetrate into the deep blind hole e.sub.2. Accordingly, a plating layer is not entirely formed on the inner circumferential surface and the bottom surface of the blind hole e.sub.2. As a result, the electrical connection between the circuit patterns c.sub.O and c.sub.2 cannot be secured.
Secondly, in the blasting step wherein the abrasive powder is blasted against the upper surface of the multilayer substrate h to form the blind holes e.sub.1 and e.sub.2 and the through hole f, since the blind holes e.sub.1 and e.sub.2 and the through hole f have different depths, it is necessary to spot-blast the abrasive powder so as to individually form the blind holes e.sub.1 and e.sub.2 and the through hole f, or repeat the formation of the mask j and the blasting of the abrasive powder every time each of the holes is formed. As a result, much time for the blasting step is required.
In the case that the difference in depth between the blind holes e.sub.1 and e.sub.2 is small, it is considered that both the blind holes e.sub.1 and e.sub.2 may be simultaneously formed because an etching rate of the insulating substrate is different from that of the copper foil. For instance, by blasting the abrasive powder to simultaneously etch the portions of the multilayer substrate h corresponding to the apertures i.sub.1 and i.sub.2 of the mask j, the blind hole e.sub.1 is first formed so that the circuit pattern c.sub.1 interposed between the insulating substrate b.sub.1 of the first layer and the insulating substrate b.sub.2 of the second layer may be exposed to the bottom of the blind hole e.sub.1, and then the blasting of the abrasive powder is continued to etch the insulating substrate b.sub.2 of the second layer until the bottom of the blind hole e.sub.2 reaches the circuit pattern c.sub.2 interposed between the insulating substrate b.sub.2 of the second layer and the insulating substrate b.sub.3 of the third layer. At this time, the bottom of the blind hole e.sub.1, that is, the circuit pattern c.sub.1 is also etched by the abrasive powder. However, since the etching rate of the circuit pattern c.sub.1 (the copper foil g.sub.1) is low, the circuit pattern c.sub.1 is not perforated but the upper surface thereof is slightly cut out. Finally, when the circuit pattern c.sub.2 becomes exposed to the bottom of the blind hole e.sub.2, the blasting of the abrasive powder is stopped. Thus, the blind holes e.sub.1 and e.sub.2 having different depths can be formed in the multilayer substrate h in such a manner that the circuit patterns c.sub.1 and c.sub.2 are exposed to the bottoms of the blind holes e.sub.1 and e.sub.2, respectively.
However, in the above case where the blind holes e.sub.1 and e.sub.2 having different depths are simultaneously formed, there is a possibility that the circuit pattern c.sub.1 exposed to the bottom of the blind hole e.sub.1 having a small depth is unduly etched or perforated under certain circumstances to cause a problem that the electrical connection by the conductive film to be formed later cannot be secured.